An Experimental Investigation Into the Reusing Sodium Contaminated Lithium Bromide Solution and Recovery of Lithium Ions by Using Electrodialysis Method

The 8th International Chemical Engineering Congress & Exhibition (IChEC 2014) Kish, Iran, 24-27 February, 2014

An experimental investigation into the reusing sodium contaminated lithium bromide solution and recovery of lithium ions by using electrodialysis method

N.Parsa, A.Moheb*, A.Mehrabani-zeinabad Department of Chemical Engineering, Isfahan University of Technology, Isfahan, Iran [email protected]

Abstract Existing a high concentration of sodium ions in the lithium bromide solution leads to becoming useless for being used as working fluid in adsorption chillers. In this work recovery of lithium hydroxide from waste stream of lithium bromide solution by batch electrodialysis was studied. A four-compartment electrodialysis cell was used for investigating the effects of applied voltage and initial feed concentration on the current density, ions permeation flux ,and ratio of lithium to sodium ions concentrations in the cathodic compartment. The obtained results showed that current density, ions permeation flux, and ratio of CLi/CNa was enhanced with increasing the applied voltage. Also with increasing the initial feed concentration, the current density and ions permeation flux increased however the ratio of CLi/CNa in the cathodic compartment decreased.

Keywords: Lithium bromide, Adsorption chiller, Batch electrodialysis, Lithium hydroxide.

Introduction Lithium bromide is widely used as operating medium for air conditioning and industrial drying system due to its hygroscopic property [1]. The biggest commercial and industrial use of lithium bromide solution is as working fluid in absorption chillers [2,3]. In LiBr/H2O systems, lithium bromide acts as absorber and water is used as refrigerant [4]. In the absorption chillers, pollution of lithium bromide solution with water leads to lowered concentration of lithiumArchive bromide and increasing the amountof of NaSID+ ions in the solution which makes the working fluid in the chillers useless. On the other hand complete separation of lithium and sodium ions is not possible because their chemical properties are similar to each other. The subject of this paper is focused for relative separation of these ions and recovery of lithium ions in the form of lithium hydroxide from waste stream of lithium bromide solution. ED is an electro chemical membrane separation process in which ions pass through an ion selective semi permeable membrane from one compartment (diluted compartment) to another (concentrated) compartment by applying an electrical field as separation driving force [5,6,7]. According to the ED principles by applying electrical potential to an ED cell containing LiBr solution, Li+ and Na+ existing in the feed solution are forced to pass through the CEM toward the cathodic compartment where they combine with hydroxide ions generated via cathodic reduction of water molecules on cathode surface to give lithium hydroxide and sodium

www.SID.ir An exprimental investigation into the reusing….. hydroxide, so the feed compartment will be depleted from these ions. By this mean, the feed solution is depleted from lithium more than sodium ions due to the following reasons:  In the ED process, the initial concentration the species affects the rate of ions transfer through the membranes. In the system under investigation in this work, the concentration of Li+ is more than Na+ in the feed solution (27800 ppm respect to 2900 ppm) and therefore Li+ transfer rate is expected to be higher than that of sodium ions.  The mass number of sodium is greater than lithium, but both of them have same charge so the mobility of sodium ions is less than lithium and lithium can pass through the CEM more rapidly than sodium ions.  By considering the atomic number (atomic number sodium is greater than lithium) and electron configuration of Li+ and Na+, mobility of Li+ is greater than Na+ so in contrast with sodium, lithium pass more rapidly. The overall result will be a lithium bromide solution in the feed compartment which is depleted from both cations. On the other hand, lithium and sodium hydroxide can be recovered in the cathodic compartment but with different concentrations.

Experimental For experiments studies a four-compartment ED cell was designed and built. The ED cell was made of plexiglas because of its proper resistance to acid, base and feed solution existing in the cell. The cell compartments were separated from each other by ion exchange membranes (CEM and/or AEM). The effective area of the membranes were 9.5*4.5 cm2 and all compartments had equal thickness of 10 mm. In this work NR cation and anion exchang membranes were used. Platinum-coated titanium and nickel mesh, both provided by Niroochlor Industry, were used as anode and cathode, respectively. The area of each electrode was 9.5*4.5 cm2. By using adjustable DC power source (Star 305, Iran) direct electrical current was applied to the cell. The experiments were run at the constant voltage. The ED experiments were conducted in the batch mode at ambient temperature. Firstly, the reservior tanks for the electrode rinse solutions were fed with 1 liter of 0.02M H2SO4 and 0.5 liter of 2.6 mM NaOH as anodic and cathodic electrode rinse solutions, respectively. Also the acid compartment reservior tank was fed with 1 liter of 0.02M HCL solution and the feed compartment reservior tank was filled with 1 liter of LiBr solution (with the different concentration for each experiment). These solution were pumped through the compartments by using proper pumps. For each experiment the reservoir tanks were filled with the fresh solutions. After the voltage was adjusted at the desired value, the process began. All experiments were run for 360 min and the samples of cathodic solution were taken from the cathodic reservoir every 120 min. After diluting the samples, concentration of Li+ and Na+ ions were determineArchived by ICP – OES device. It should of be mentioned SID that at first a solution of 2.6 mM NaOH as the cathodic electrod rinse to obtain a solution containing LiOH. After the first experiment done, cathodic solution of the first experiment was maintained as mother solution to make cathodic electrode rinse for other experiments. This solution was diluted 88 times and 500cm3 of it was used for each experiment as the cathodic electrod rinse solution.

Results and Discussion The following equation can be used for determining of permeation flux of Li+ ions.

J= ( (1)

+ Where in this equation Vc is the cathodic solution volume (lit), Ct is the concentration of Li (mol/lit) in the cathodic compartment at time t whereas C0 is the initial concentration at time

www.SID.ir The 8th International Chemical Engineering Congress & Exhibition (IChEC 2014) Kish, Iran, 24-27 February, 2014 zero. A presents the effective area (m2) of the membrane and t is the operation time (min). Using this equation J is obtained in mol/m2.min. In order to investigate the effect of applied voltage, the experiments were carried out for 360 min using diluted cathodic solution.The limiting current is an important parameter in the ED process which determines the electrical resistance and current utilization [7]. If the plot of voltage against current shows a point of inflection, the limiting current is reached and this event is called concentration polarization. In the ED process current must be kept under limiting current [8,9]. Fig. 1. shows time averaged electrical current density versus applied voltage for different feed concentrations. It is seen in this figure that for all values of feed concentrations the current density increased with increasing the applied voltage. In other word the current density did not reach the limiting current density and no concentration polarization was occurred in any case. Also ratio of lithium to sodium ions concentration in the cathodic compartment for different applied voltage is given in Fig. 2. It was seen that ratio of CLi/CNa increased with increasing applied voltage. In order to study the effect of feed concentration, experiments were performed with two different concentrations of lithium ions in the feed solution. (Cf:27800 ppmLi+, Cf/2:13900 ppmLi+). According to Fig. 3. for the applied voltage of 7V, by increasing the feed concentration, the cell current density increased. This can be explained by the fact that at the higher concentration, the total number of the ions and consequently conductivity in the feed compartment are augmented. So the electrical resistance of this compartment and as a result the cell total electrical resistance is reduced. At the constant voltage, when the resistance of the cell is reduced the current density should be increased [8,9]. The ratio of lithium to sodium ions concentration in the cathodic compartment at the constant applied voltage for different feed concentrations is given in Fig. 4. Also Fig. 5. shows variation of lithium ions permeation flux versus feed concentration at diffrent voltages. By increasing the feed concentration the lithium ions permiation flux is increased. This can be explained by the fact that when the applied voltage is increased the electrical potential and consequently diving force for transferring the ions is enhanced so lithium ions permiation flux increased. 80

30 v=3,Cf ) 2 Cf 60 v=5,Cf Cf/2 50 v=7,Cf 20 40 30

Sodium Ions Ions Sodium 20 Current Current 10 Concentration

Ratio of Lithium to to of Lithium Ratio 10 0 Density(mAmp/cm 0 0 100 200 300 400 3 4 Archive5 6 7 8 of SIDTime(min) Voltage(volt)

Fig. 1. Variaton of averaged current density with fig. 2. Variation of ratio of lithium to sodium applied voltage ions concentration with applied voltage Conclusions In this work, batch ED process was used for recovery of lithium hydroxide in the cathodic compartment from waste stream of lithium bromide solution contaminated with sodium ions. Effects of some operating parameters such as applied voltage, initial feed concentration on the recovery of lithium hydroxide were investigated. It was seen that in the range of parameters under investigation in this study high ratio of CLi/CNa and high permeation flux of lithium ions could be obtained by using the proposed process. At high applied voltage and low feed concentration, ratio of CLi/CNa increased and high permeation flux occurred at the high feed

www.SID.ir An exprimental investigation into the reusing….. concentration and high applied voltage.

0.4 v=7 0.3

.min) v=5 2 0.2 v=3

0.1 J(mol/m 0 0 5000 10000 15000 20000 25000 30000 LiBr Initial Concentration (ppm)

Fig. 5. Variation of lithium ions permeation flux with feed concentration

100

) 50

2 v=7, Cf

v=7,Cf 80 40 v=7,Cf/2 v=7,Cf/2 60 30

40 20 Current Current

Sodium Ions Ions Sodium 10

Concentration 20 Ratio of Lithium to to of Lithium Ratio

Density(mAmp/cm 0 0 0 100 200 300 400 0 100 200 300 400 Time(min) Time(min)

fig. 4. Variation of ratio of lithium to sodium Fig. 3. Variation of averaged current density ions concentration with feed concentration with feed concentration

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